Short answer. Yes to both. The law expressly allows two or more persons to be substituted for one heir, and one person to be substituted for two or more heirs. You are free to arrange the numbers on each side of a substitution as your plan requires; it need not be one substitute for one heir.
What the law says
Two or more persons may be substituted for one; and one person for two or more heirs.
Civil Code, Article 860 — Brief or Compendious Substitution. Read the full provision →
The numbers are flexible
Article 860 is short and permissive: two or more persons may be substituted for one; and one person for two or more heirs. A substitution does not have to be a neat pairing. You can appoint several people to share what a single instituted heir would have taken, or appoint one person to catch the shares of several instituted heirs if they fall away. This kind of arrangement, sometimes called a brief or compendious substitution, simply gives you freedom in how the backup layer is structured, matching it to your family and intentions rather than to a rigid formula.
Several substitutes for one heir
Where you name more than one person to substitute for a single heir, they take the vacant share together. Unless you have indicated some other division, they share it equally between them, each receiving a like portion of what the original heir would have received. If you want them to take in unequal proportions, that has to be stated, because absent instructions the law distributes the share among them evenly. Either way, they collectively step into the place the instituted heir would have held, receiving only what that heir's share amounted to.
One substitute for several heirs
The mirror arrangement lets a single person stand behind a group of instituted heirs. If any of those heirs dies before you, refuses, or is incapacitated, the substitute takes that heir's share; if more than one of them falls away, the substitute takes each of the vacated shares. This is a convenient way to name one trusted fallback for a whole set of primary beneficiaries without having to pair a separate substitute with each. The substitute's entitlement rises or falls with how many of the instituted heirs actually drop out.
Spell out the shares you intend
The freedom in numbers makes clear drafting worth the effort. Say who substitutes for whom, and if several substitutes are not to share equally, or the shares are to pass in some particular way, put that in the will. The default of equal division fills gaps the testator leaves, and it may not match what you actually want. Setting the proportions out expressly avoids a later dispute over how a substituted share is to be split, and a lawyer can capture the arrangement precisely on the page.